Microfibrillated cellulose, hereinafter denoted MFC, is also called nanocellulose. MFC is prepared from cellulose source material, usually from woodpulp. The MFC fibrils are isolated from the wood-based fibers using high-pressure homogenizers. The homogenizers are used to delaminate the cell walls of the fibers and liberate the microfibrils and/or nanofibrils. Pre-treatments are sometimes used to reduce the high energy consumption. Examples of such pretreatments are enzymatic/mechanical pre-treatment and introduction of charged groups e.g. through carboxymethylation or TEMPO-mediated oxidation.
The properties of MFC, such as mechanical properties, film-forming properties and viscosity, makes it an interesting material for many applications, such as paper and board manufacturing, oil and mining, composites technology, food industry, pharmaceutical industry, cosmetic applications etc.
Following documents of the prior art relates to the production of MFC:
U.S. Pat. No. 4,483,743 discloses a process for manufacturing microfibrillated cellulose (MFC) by passing a liquid suspension of cellulose through a high pressure homogenizer having a small diameter orifice in which the suspension is subjected to a pressure drop of at least 3000 psig (20670 kPa) and a high velocity shearing action followed by a high velocity decelerating impact, and repeating the passage of said suspension through the orifice until the cellulose suspension becomes substantially stable. The produced MFC has a water retention value of over 280%. The MFC can be used with paper products and non-woven sheets to improve their strength. MFC produced by this type of process typically has a width of about 25-100 nm while the length is much longer.
WO 2007/091942 A1 discloses an improved method for manufacturing microfibrillated cellulose. The disclosed method is said to solve the problems relating to clogging in high-pressure homogenizers and high energy consumption. According to this document microfibrillated cellulose is manufactured by refining a hemicelluloses containing pulp, preferably sulphite pulp, and treating the pulp with a wood degrading enzyme followed by homogenizing the pulp. The enzyme is a cellulase, preferably a cellulase of endoglucanase type which most preferably is a mono-component endoglucanase. The pulp can be refined before or after the enzyme treatment or both before and after the enzyme treatment. The obtained microfibrillated cellulose can be used in food products, cosmetic products, pharmaceutical products, paper products, composite materials, coatings or in rheology modifiers (e.g. drilling muds).
Yet another type of microfibrillated cellulose is described by Wågberg Lars et al., Langmuir 2008, Vol. 24, 2008, pages 784-795. This microfibrilled cellulose was prepared by high-pressure homogenization of carboxymethylated cellulose fibers. The fibers were sulfite softwood-dissolving pulp fibers. The produced MFC typically has a width of about 5-15 nm and a length which can be more than 1 μm.
Also other chemical pretreatment methods are known, such as an oxidation pretreatment of pulp fibers described by Saito et al. in Biomacromolecules, Vol. 8, No. 8, 2007, pp. 2485-2491. The pulp fibers are oxidized with a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated system followed by mechanical treatment. This oxidation pretreatment converts primary hydroxyl groups of the celluloses to carboxylate groups. The produced nanofibers typically have a width of about 3-4 nm and a length of a few μm.
Following documents of the prior art relate to drying/dewatering MFC or other cellulosic materials:
When increasing the dry content of MFC suspensions, irreversible aggregation phenomenons have been recorded. The main strategy to prevent agglomeration or hornification of MFC has been the introduction of a steric barrier or electrostatic groups to block cooperative hydrogen bonding of the cellulose chains. Among the most useful additives are polyhydroxy-functionalized admixtures, particularly carbohydrates or carbohydrate related compounds, such as glycosides, carbohydrate gums, cellulose derivatives, e.g. CMC, starches, and oligosaccharides. Unfortunately, large quantities of such substances seem to be necessary to prevent hornification. This might severely restrict the final use of the MFC, for example in food applications, composite materials etc.
WO 2012/107642 discloses a method for dewatering an aqueous gel of nanofibrillar cellulose by means of an organic solvent miscible with water. The preferred solvent is ethanol. A dry content of the nanofibrillar cellulose of up to 95% is claimed to be achieved.
WO 2005/028752 discloses a method of making a formed, dried fiber material. The starting material is an aqueous lignocellulose fiber pulp which is dewatered under an effective compression direction and pressure, and then dried in a drying oven at a temperature of 60-120° C. In a preferred embodiment aqueous lignocellulose fiber material is pumped into a formation trough having fixed, non-perforated upper side plates, a removable perforated bottom, a mechanically driven, perforated or solid plunger top and mechanically driven, solid lower side plates, and allowed to dewater.
WO 2011/095335 discloses a method for producing dry microfibrillated cellulose, wherein a mixture of cellulose pulp in a liquid such as ethanol, is subjected to high shear actions to form a MFC slurry, then the ethanol in the MFC liquid is displaced by liquid carbon dioxide under pressure and the liquid carbon dioxide is removed by evaporation to obtain dry MFC.
JP 60186548 discloses a method for dehydrating and drying microfibrilled cellulose by adding to an aqueous suspension of MFC a water-soluble substance, such as glucose or sucrose, in an amount of at least 10% by weight of the solid component of the MFC. The obtained MFC composition is said to have excellent redispersing and suspending properties.
U.S. Pat. No. 4,481,076 describes a method of drying an aqueous suspension of microfibrillated cellulose by suspending the same in a compound capable of inhibiting hydrogen bonding between the fibrils in the cellulose and drying the suspension, preferably at a temperature of 50°-70° C. Preferred compounds are polyhydroxy compounds such as sugars.
The present invention is focussed on solving following problems:
MFC is normally produced in very low solid content, usually at a consistency of between 1% and 6% by weight. Higher solid content is needed for more feasible transportation and further processing.
When increasing the dry content irreversible agglomeration or hornification occurs, which makes redispersion after drying difficult.
Thus, the aim of the invention is to obtain a feasible process which prevents agglomeration during drying, and to obtain a MFC product which has a high solids content and which is redispersible.